For example, solar heat collected through the south-facing windows of Denmark takes care of more than 10 per cent of all the thermal-energy requirement of the country.  During the hot season in a cold region, every square metre of such a `solar-window’ contributes more than 200 kwH of energy.

The basic idea in passive solar heating is designing the house in a manner wherein the structure absorbs the sun’s heat in order to utilize it even after the sunset, during the frosty nights.

Trapping the Sun

 A passive solar house utilizes its own architecture to trap the sun.  The design itself synchronizes with the mini-biosphere of the site.  The structure is well-oriented towards a maximum yield of solar-heat during the cold periods.

Windows collect the heat from the sun, along with relfectors, skylights and appropriately designed roofs and walls.  In fact, specially constructed floors and brickwalls absorb heat to release it later when needed.  It is essential to close all heat collecting apertures at night to check the heat loss.

Conventionally speaking, a solar heating plant consists of a solar collector, a heat transport system and a heat store.  All these three parts form an integral and inherent component of the architecture of the passive solar house.

Collecting the Sunshine

You must have witnessed that cars standing out in the sunlight as well as rooms with covered glass windows get heated up by the sunrays streaming through the window.  In fact, a crystal clear glass allows up to 90 percent of solar energy, absorbing or reflecting the rest of it.  Transparency of the glass determines the depth of solar penetration.  Actually, glass is the most common and simplest `flat-plate’ solar heat collector.  Its transparent outer layer is common to all thermal solar collectors.  If this layer is a glass-window, it will overheat the interior of the room.  But if such a layer is in front of a wall then the wall will soak the heat throughout te day and release it (to the interior) during the night.

Soaking the Sun

A transparent cover plate in front of a black absorbent material and an insulated wall is a highly efficient flat-plate solar collector.  With the air-flow behind the transparent plate, the collector’s covering layer and the absorber (a thin metal plate) provide a combined  insulation equal to that provided by two layers of glass.

The efficiency of a collector is directly proportional to the convertibility of the radiated energy into useful thermal energy.  We could achieve an optimal thermal efficiency by having a low working temperature inside the collectors, minimum heat loss through the cover and maximum heat-absorption by the absorber.

Transporting Solar Energy

Heat from the solar-collector is either transferred straight to the point of use or else stored.  This transfer takes place to the `heat-store’ along ventilation ducts.  The heat is transported from the store to walls or floors of the house, which in turn heats up the inside space.

In a passive solar house heat is stored by building components having high heat-storage capacity-like stone walls and tiled floors.  Even a room filled with crushed stones could act as a heat-store.  In fact, a Trombe wall is an excellent heat-store, operating at temperature ranging from 40 to 80 degrees centigrade.

The Trombe Wall

Trombe walls are solar-collectors, with openings at the bottom and top to facilitate the passage of the hot air into the interior rooms.  Thus, the air in the front circulates from the front of the wall to heat up the room at the back.  This principle of Trombe Walls is optimal for cold regions of North India (like Himachal Pradesh) where we have warm days and freezing nights.

Such “heat-walls” could be built with either compacted earth or mud blocks, which could be prepared by utilising TARA Balram (mud block) Press - a Development Alternatives’ innovation.  Basically, the thickness of a Trombe wall is such that it retains the heat for one full day, before transporting it to  the interior in the cold hours of the night.

Solar Insulation

“Using transparent insulation, it will be possible to save up to 40 percent of the fuel required for heating in correctly planned buildings in cold climates”, explains Friti Salvesen - a solar scientist from Norway.

Plastic-based insultation is the `in-thing’ nowadays in terms of passive solar heating.  This popular insulation material comprises a honey-comb structure of poly-carbonate foil, with a glass or plastic sheet on either side.

This transparent sheet acts as a solar collector cover to heat up the air behind the cover.  It is also used when the atmospheric temperature is low-like that in the hilly regions.  This material has proved to be optimum insulation in passive solar energy systems like Trombe Walls, solar conservatories, integrated solar collectors and heat stores.

A Closed glass-window, however, provides the simplest and yet effective transparent insulation.  Apart from its easy accessibility, glass is a durable and environment-friendly substance.  Hence, even the common mountain-folk of India could afford a passive solar house built with local (solar-friendly) material.

Free Heating

Basically, a solar air-heating system (in a passive solar house) creates a warmer climate around the building and supplies solar heat to inside rooms as well.

Scientists predict that solar energy will be capable of providing future buildings with almost free heating.  The air-heating system is inexpensive and fits easily into walls and roofs.  At the same time, it is built from local building materials with local manpower.  Hence, it is truly an appropriate technology for the forestry hills and cold deserts of India.

Source: The Sunshine Revolution

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